Weight-set pack-off units are well known in the art for sealing the annulus between an inner and an outer pipe, such as a mandrel casing hanger and a wellhead body in a well. Examples of such pack-off units are shown in U.S. Pat. No. 3,268,241 to Castor et al.; U.S. Pat. No. 3,972,546 to Putch; and U.S. Pat. No. 4,138,144 to Pierce, Jr.
However, there are various disadvantages with the known prior art pack-off units. In the first place, the upward load capacity of these units in the event that the unit seals pressure from the bottom of the well is limited, mostly due to the locking mechanism of the pack-off unit to the outer wellhead body. While various types of pipe or pack-off locking mechanisms are known in the art, these are usually complicated to manufacture and use. Examples are shown in U.S. Pat. No. 3,404,736 to Nelson et al.; U.S. Pat. No. 3,457,992 to Brown; and U.S. Pat. No. 3,789,925 to Brown. In addition, the pack-off units of the prior art have a limited capacity to carry downward loads, with the typical configuration having the load carried through the weight-set resilient packing.